Abstract Mucopolysaccharidosis type I (MPS I), resulting from the deficiency of alpha-L- iduronidase (IDUA), is one of the most common lysosomal storage diseases (LSD) affecting the central nervous system (CNS), which cannot be cured by current treatments. We have shown that lentiviral vector (LV)-mediated gene delivery can produce sustained supraphysiological IDUA levels in plasma and may be therapeutic in treating CNS manifestations if the blood-brain barrier (BBB) can be overcome. The utilization of the endogenous receptor-mediated transcytosis system on BBB-forming brain capillary endothelium will enable rapid and wide delivery of neurotherapeutics across the BBB via circulation. The overall goal of the work proposed is to develop a novel therapeutic approach utilizing low-density lipoprotein receptor family (LDLRf)- mediated transcytosis for fusion protein delivery across the BBB via LV-mediated gene transfer in the liver and/or the HSC for the treatment of CNS manifestations in MPS I. We will identify, by in vitro and in vivo evaluation, the optimal LDLRf-binding domain of apoE for most efficient BBB transport while retaining the normal catalytic function and lysosomal enzyme trafficking of fusion IDUA (Specific Aim 1). The spatial and temporal protein distribution profile will be studied in the CNS and peripheral organs, targeting the liver or HSC-derived erythroid cells as depot organ for tissue-specific transgene production in mice with different ages (Specific Aim 2). Preclinical evaluation will be conduced in a murine MPS I model to identify window of CNS treatment by GAG assay for metabolic correction, pathology evaluation for CNS normalization and behavioral assessments for improvement of CNS functional deficits (Specific Aim 3). Taken together, these studies will not only lead to the development of a novel approach for the treatment of neurological diseases, such as in MPS I, with lifelong BBB-targeted protein delivery, but also provide important knowledge of in vivo changes in receptor-mediated BBB transport system with aging or under pathological conditions.